Muscarinic induction of hippocampal gamma oscillations requires coupling of the M1 receptor to two mixed cation currents

Acetylcholine modulates cortical synaptic transmission via different muscarinic receptors, as studied with receptor knockout mice

The central cholinergic system plays a crucial role in synaptic plasticity and spatial attention; however, the roles of the individual cholinergic receptors involved in these activities are not well understood at present. In the present study, we show that acetylcholine (ACh) can facilitate or depress synaptic transmission in occipital slices of mouse visual cortex. The precise nature of the ACh effects depends on the ACh concentration, and is input specific, as shown by stimulating different synaptic pathways. Pharmacological blockade of muscarinic receptor (mAChR) subtypes and the use of M1-M5 mAChR-deficient mice showed that specific mAChR subtypes, together with the activity of the cholinesterases (ChEs), mediate facilitation or depression of synaptic transmission. The present data suggest that local ACh, acting through mAChRs, regulates the cortical dynamics making cortical circuits respond to specific stimuli.

Regulation of recombinant and native hyperpolarization-activated cation channels

Ionic currents generated by hyperpolarization-activated cation-nonselective (HCN) channels have been principally known as pacemaker h-currents (Ih), because they allow cardiac and neuronal cells to be rhythmically active over precise intervals of time. Presently, these currents are implicated in numerous additional cellular functions, including neuronal integration, synaptic transmission, and sensory reception. These roles are accomplished by virtue of the regulation of Ih by both voltage and ligands. The article summarizes recent developments on the properties and allosteric interactions of these two regulatory pathways in cloned and native channels. Additionally, it discusses how the expression and properties of native channels may be controlled via regulation of the transcription of the HCN channel gene family and the assembly of channel subunits. Recently, several cardiac and neurological diseases were found to be intimately associated with a dysregulation of HCN gene transcription, suggesting that HCN-mediated currents may be involved in the pathophysiology of excitable systems. As a starting point, we briefly review the general characteristics of Ih and the regulatory mechanisms identified in heterologously expressed HCN channels.

Cholinergic Stimulation of Amylase Secretion from Pancreatic Acinar Cells Studied with Muscarinic Acetylcholine Receptor Mutant Mice

Muscarinic acetylcholine receptors (mAChRs) expressed by pancreatic acinar cells play an important role in mediating acetylcholine-dependent stimulation of digestive enzyme secretion. To examine the potential roles of M(1) and M(3) mAChRs in this activity, we used M(1) and M(3) receptor single knockout (KO) and M(1)/M(3) receptor double KO mice as novel experimental tools. Specifically, we examined the ability of the muscarinic agonist carbachol to stimulate amylase secretion in vitro, using dispersed pancreatic acini prepared from wild-type and mAChR mutant mice. Quantitative reverse transcription-polymerase chain reaction studies using RNA prepared from mouse pancreatic acini showed that deletion of the M(1) or M(3) mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Moreover, immunoprecipitation studies with M(1) and M(3) mAChR-selective antisera demonstrated that both mAChR subtypes are expressed by mouse pancreatic acini. Strikingly, carbachol-induced stimulation of amylase secretion was significantly impaired in acinar preparations from both M(1) and M(3) receptor single KO mice and abolished in acinar preparations from M(1)/M(3) receptor double KO mice. However, another pancreatic secretagogue, bombesin, retained its ability to fully stimulate amylase secretion in acinar preparations from M(1)/M(3) receptor double KO mice. Together, these studies support the concept that cholinergic stimulation of pancreatic amylase secretion is mediated by a mixture of M(1) and M(3) mAChRs and that other mAChR subtypes do not make a significant contribution to this activity. These findings clarify the long-standing question regarding the molecular nature of the mAChR subtypes mediating the secretion of digestive enzymes from the exocrine pancreas.

A behavioral role for dendritic integration: HCN1 channels constrain spatial memory and plasticity at inputs to distal dendrites of CA1 pyramidal neurons

The importance of long-term synaptic plasticity as a cellular substrate for learning and memory is well established. By contrast, little is known about how learning and memory are regulated by voltage-gated ion channels that integrate synaptic information. We investigated this question using mice with general or forebrain-restricted knockout of the HCN1 gene, which we find encodes a major component of the hyperpolarization-activated inward current (Ih) and is an important determinant of dendritic integration in hippocampal CA1 pyramidal cells. Deletion of HCN1 from forebrain neurons enhances hippocampal-dependent learning and memory, augments the power of theta oscillations, and enhances long-term potentiation (LTP) at the direct perforant path input to the distal dendrites of CA1 pyramidal neurons, but has little effect on LTP at the more proximal Schaffer collateral inputs. We suggest that HCN1 channels constrain learning and memory by regulating dendritic integration of distal synaptic inputs to pyramidal cells.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

The kainate receptor subunit GluR6 mediates metabotropic regulation of the slow and medium AHP currents in mouse hippocampal neurones

Kainate receptors (KARs) play an important role in synaptic physiology, plasticity and pathological phenomena such as epilepsy. However, the physiological implications for single cells and neuronal networks of the distinct expression patterns of KAR subunits are unknown. One intriguing effect of KAR activation is a long-term change to intrinsic neuronal excitability and neuronal firing patterns, such as single-spike and spike-burst firing. In this study, we describe the role of kainate receptor subunits in the metabotropic regulation of the slow and medium afterhyperpolarization (AHP) currents (I(sAHP), I(mAHP)). Using whole-cell patch-clamp recordings from CA3 pyramidal cells of wild-type (WT) and KAR knockout mice, we show that the kainate-induced decrease of I(sAHP) and I(mAHP) amplitude is protein-kinase-C-dependent and absent in GluR6(-/-) but not GluR5(-/-) pyramidal neurones. Our findings suggest that activation of GluR6-containing KARs modulates AHP amplitude, and influences the firing frequency of pyramidal neurones.

Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro

Gamma frequency network oscillations are assumed to be important in cognitive processes, including hippocampal memory operations, but the precise functions of these oscillations remain unknown. Here, we examine the cellular and network mechanisms underlying carbachol-induced fast network oscillations in the hippocampus in vitro, which closely resemble hippocampal gamma oscillations in the behaving rat. Using a combination of planar multielectrode array recordings, imaging with voltage-sensitive dyes, and recordings from single hippocampal neurons within the CA3 gamma generator, active current sinks and sources were localized to the stratum pyramidale. These proximal currents were driven by phase-locked rhythmic inhibitory inputs to pyramidal cells from identified perisomatic-targeting interneurons. AMPA receptor-mediated recurrent excitation was necessary for the synchronization of interneuronal discharge, which strongly supports a synaptic feedback model for the generation of hippocampal gamma oscillations.

Mechanisms underlying gamma ('40 Hz') network oscillations in the hippocampus--a mini-review

Gamma-frequency oscillations (approximately 30-100 Hz) in cortical network activity have been proposed to provide a temporal structure for various forms of cognitive processing. This review provides an update on recent experiments addressing the mechanisms underlying gamma-frequency network oscillations in the rodent hippocampus. Particular emphasis is placed on the correlation between in vivo observations and in vitro models.

Distinct properties of carbachol- and DHPG-induced network oscillations in hippocampal slices

The aim of this study was to compare and contrast the properties of gamma oscillations induced by activation of muscarinic acetylcholine or metabotropic glutamate receptors in the CA3 region of rat hippocampal slices. Both carbachol and the group I metabotropic glutamate receptor agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), induced network oscillations in the gamma-frequency range (30-100 Hz). The M1 muscarinic receptor antagonist, pirenzepine, blocked carbachol-, but enhanced DHPG-induced oscillations, whereas LY 341495, an antagonist at metabotropic glutamate receptors, abolished DHPG-, but left carbachol-induced oscillations unchanged. There were significant differences in the peak frequency, maximal power, and spectral width of the two oscillations. Pharmacological experiments showed that both types of oscillation depend on fast excitatory and inhibitory synaptic transmission. Interestingly, activation of neurokinin-1 receptors by substance P fragment or enhancement of inhibitory synaptic currents by the benzodiazepine ligand, zolpidem, boosted DHPG-, but reduced the power of carbachol-induced oscillations. These results suggest that, although carbachol and DHPG might activate similar conductances in individual pyramidal cells, the oscillations they induce in slices involve different network mechanisms, most likely by recruiting distinct types of GABAergic interneuron.

Cholinergic modulation of hippocampal cells and circuits

Septo-hippocampal cholinergic fibres ramify extensively throughout the hippocampal formation to release acetylcholine upon a diverse range of muscarinic and nicotinic acetylcholine receptors that are differentially expressed by distinct populations of neurones. The resultant modulation of cellular excitability and synaptic transmission within hippocampal circuits underlies the ability of acetylcholine to influence the dynamic properties of the hippocampal network and results in the emergence of a range of stable oscillatory network states. Recent findings suggest a multitude of actions contribute to the oscillogenic properties of acetylcholine which are principally induced by activation of muscarinic receptors but also regulated through activation of nicotinic receptor subtypes.

Kainate receptors and rhythmic activity in neuronal networks: hippocampal gamma oscillations as a tool

Rhythmic electrical activity is ubiquitous in neuronal networks of the brain and is implicated in a multitude of different processes. A prominent example in the healthy brain is electrical oscillations in the gamma-frequency band (20-80 Hz) in hippocampal and neocortical networks, which play an important role in learning, memory and cognition. An example in the pathological brain is electrographic seizures observed in certain types of epilepsy. Interestingly the activation of kainate receptors (KARs) plays an important role in synaptic physiology and plasticity, and can generate both gamma oscillations and electrographic seizures. Electrophysiological recordings of extracellular gamma oscillations and intracellular currents in a hippocampal slice combined with computer modelling can shed light on the expression loci of KAR subunits on single neurones and the distinct roles subunits play in rhythmic activity in the healthy and the pathological brain. Using this approach in wild-type (WT) and KAR knockout mice it has been shown that KAR subunits GluR5 and GluR6 have similar functions during gamma oscillations and epileptiform bursts and that small changes in the overall activity in the hippocampal area CA3 can tilt the balance between excitation and inhibition and cause the neuronal network to switch from gamma oscillations to epileptiform bursts.

Distinct roles for the kainate receptor subunits GluR5 and GluR6 in kainate-induced hippocampal gamma oscillations

Kainate receptors (KARs) play an important role in synaptic physiology, plasticity, and pathological phenomena such as epilepsy. However, the physiological implications for neuronal networks of the distinct expression patterns of KAR subunits are unknown. Using KAR knock-out mice, we show that subunits glutamate receptor (GluR) 5 and GluR6 play distinct roles in kainate-induced gamma oscillations and epileptiform burst activity. Ablation of GluR5 leads to a higher susceptibility of the network to the oscillogenic and epileptogenic effects of kainate, whereas lack of GluR6 prevents kainate-induced gamma oscillations or epileptiform bursts. Based on experimental and simulated neuronal network data as well as the consequences of GluR5 and GluR6 expression for cellular and synaptic physiology, we propose that the functional interplay of GluR5-containing KARs on axons of interneurons and GluR6-containing KARs in the somatodendritic region of both interneurons and pyramidal cells underlie the oscillogenic and epileptogenic effects of kainate.

Cellular mechanisms of neuronal population oscillations in the hippocampus in vitro

A variety of population oscillations, at frequencies approximately 5 Hz up to 200 Hz and above, can be induced in hippocampal slices either by (a) manipulation of the ionic environment, or (b) by stimulation of metabotropic receptors; brief oscillations can even occur spontaneously. In this review, we consider in vitro theta (4-12 Hz), gamma/beta (15-70 Hz), and very fast oscillations (VFO) (>70 Hz). Many in vitro oscillations are gated by synaptic inhibition but are influenced by electrical coupling as well; one type depends solely on electrical coupling. For some oscillations dependent upon inhibition, the detailed firing patterns of interneurons can influence long-range synchronization. Two sorts of electrical coupling are important in modulating or generating various in vitro oscillations: (a) between interneurons, primarily between dendrites; and (b) between axons of pyramidal neurons. VFO can exist in isolation or can act as generators of gamma frequency oscillations. Oscillations at gamma frequencies and below probably create conditions under which synaptic plasticity can occur, between selected neurons-even those separated by significant axonal conduction delays.

Cholinergic Stimulation of Salivary Secretion Studied with M1 and M3 Muscarinic Receptor Single- and Double-Knockout Mice

Identification of the specific muscarinic acetylcholine receptor (mAChR) subtypes mediating stimulation of salivary secretion is of considerable clinical interest. Recent pharmacological and molecular genetic studies have yielded somewhat confusing and partially contradictory results regarding the involvement of individual mAChRs in this activity. In the present study, we re-examined the roles of M(1) and M(3) mAChRs in muscarinic agonist-mediated stimulation of salivary secretion by using M(1) and M(3) receptor single-knockout (KO) mice and newly generated M(1)/M(3) receptor double-KO mice. When applied at a low dose (1 mg/kg, s.c.), the muscarinic agonist pilocarpine showed significantly reduced secretory activity in both M(1) and M(3) receptor single-KO mice. However, when applied at higher doses, pilocarpine induced only modestly reduced (5 mg/kg, s.c.) or unchanged (15 mg/kg, s.c.) salivation responses, respectively, in M(1) and M(3) receptor single-KO mice, indicating that the presence of either M(1) or M(3) receptors is sufficient to mediate robust salivary output. Quantitative reverse transcriptase-polymerase chain reaction studies with salivary gland tissue showed that the inactivation of the M(1) or M(3) mAChR genes did not lead to significantly altered mRNA levels of the remaining mAChR subtypes. Strikingly, the sialagogue activity of pilocarpine was abolished in M(1)/M(3) receptor double-KO mice. However, salivary glands from M(1)/M(3) receptor double-KO mice remained responsive to stimulation by the beta-adrenergic receptor agonist, (S)-isoproterenol. Taken together these studies support the concept that a mixture of M(1) and M(3) receptors mediates cholinergic stimulation of salivary flow.

Input resistance is voltage dependent due to activation of Ih channels in rat CA1 pyramidal cells

The contribution of the hyperpolarization-activated cation current (I(h)) to input resistance (R(N)) and resting potential (RP) was investigated during whole-cell patch-clamp recordings in CA1 pyramidal cells of rat hippocampal slices. In current-clamp mode, R(N) was determined at different membrane potentials. R(N) decreased with increasing hyperpolarization, from about 260 Momega to 140 Momega at potentials of about -60 mV and -110 mV, respectively. Both the potential of half-maximal reduction of R(N) and the potential of half-maximal I(h) activation (determined in voltage-clamp mode) were approximately -90 mV. The analysis of the voltage sag indicative of I(h) activation revealed a preferential activity of I(h) channels in a voltage range between -70 and -95 mV. ZD7288 (50 microM), a specific I(h) blocker, led to a hyperpolarization by about 4.8 mV, increased R(N) by approximately 45% within a potential range between -65 and -80 mV, and abolished the voltage dependence of R(N). Gabapentin (GBP, 100 microM), an I(h) channel agonist, led to a depolarization by about 2.4 mV and reduced R(N) by about 20% within a potential range between -65 and -80 mV. In conclusion, our data show that R(N) is voltage dependent due to I(h) channel activation and that I(h) channels are preferentially active at voltages between -70 and -95 mV. Furthermore, we demonstrated that R(N) can be modulated by antiepileptic drugs such as GBP, which may partly explain its antiepileptic effect as due to decreasing the sensitivity to excitatory input.

Muscarinic acetylcholine receptor knockout mice: novel phenotypes and clinical implications

Muscarinic acetylcholine receptors (mAChRs; M1-M5) play key roles in regulating the activity of many important functions of the central and peripheral nervous system. Because of the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues or cell types express two or more mAChR subtypes, identification of the physiological and pathophysiological roles of the individual mAChR subtypes has proven a difficult task. To circumvent these difficulties, several laboratories recently employed gene-targeting techniques to generate mutant mouse strains deficient in each of the five mAChR subtypes. Phenotyping studies showed that each mutant mouse line displayed characteristic physiological, pharmacological, behavioral, biochemical, or neurochemical deficits. The novel insights gained from these studies should prove instrumental for the development of novel classes of muscarinic drugs.

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.

Role of muscarinic receptor subtypes in the constriction of peripheral airways: studies on receptor-deficient mice

In the airways, increases in cholinergic nerve activity and cholinergic hypersensitivity are associated with chronic obstructive pulmonary disease and asthma. However, the contribution of individual muscarinic acetylcholine receptor subtypes to the constriction of smaller intrapulmonary airways that are primarily responsible for airway resistance has not been analyzed. To address this issue, we used videomicroscopy and digital imaging of precision-cut lung slices derived from wild-type mice and mice deficient in either the M1 (mAChR1-/- mice), M2 (mAChR2-/- mice), or M3 receptor subtype (mAChR3-/- mice) or lacking both the M2 and M3 receptor subtypes (mAChR2/3-/- double-knockout mice). In peripheral airways from wild-type mice (mAChR+/+ mice), muscarine induced a triphasic concentration-dependent response, characterized by an initial constriction, a transient relaxation, and a sustained constriction. The bronchoconstriction was diminished by up to 60% in mAChR3-/- lungs and was completely abolished in mAChR2/3-/- lungs. The sustained bronchoconstriction was reduced in mAChR2-/- bronchi, and, interestingly, the transient relaxation was absent; the bronchoconstriction in response to 10-8 M muscarine was increased by 158% in mAChR1-/- mice. Quantitative reverse transcriptase-polymerase chain reaction analysis revealed that the disruption of specific mAChR genes had no significant effect on the expression levels of the remaining mAChR subtypes. These results demonstrate that cholinergic constriction of murine peripheral airways is mediated by the concerted action of the M2 and M3 receptor subtypes and suggest the existence of pulmonary M1 receptor activation, which counteracts cholinergic bronchoconstriction. Given the important role of muscarinic cholinergic mechanisms in pulmonary disease, these findings should be of considerable therapeutic relevance.

Muscarinic acetylcholine receptor subtypes in cerebral cortex and hippocampus

The M1, M2 and M4 subtypes of mAChRs are the predominant receptors in the CNS. These receptors activate a multitude of signaling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of ACh release. In addition, novel functions mediated by mAChRs are currently being discovered. These studies are greatly facilitated by the recent development of subtype selective toxins and mice lacking individual mAChR genes. Studies in cell culture and the rodent brain demonstrate that mAChR internalization and intracellular trafficking is an important component of mAChR regulation. Characterizing mAChR intracellular trafficking could help facilitate the development of selective mAChR ligands. For example, a selective M1 agonist would cause a shift in the distribution of M1 from the cell surface to an intracellular distribution, while M2 and M4 would remain on the cell surface. Characterizing mAChR intracellular trafficking is also important for understanding the cellular mechanisms that regulate mAChR cell surface expression and signaling. Furthermore, intracellular trafficking has recently been demonstrated to play a role in the development of tolerance to drugs (Whistler et al., 1999; He et al., 2002). Because individual mAChR subtypes are novel targets for treatments of diseases such as Alzheimer's disease and schizophrenia, understanding the mechanisms that regulate mAChR signaling and intracellular trafficking following acute and chronic stimulation might lead to the development of rational strategies.

The sleep-modulating peptide cortistatin augments the h-current in hippocampal neurons

Cortistatin (CST) is a sleep-modulating peptide found exclusively in the brain. Although CST is closely related to somatostatin (SST) and binds to SST receptors, CST has effects on sleep and neuronal activity in cortex and hippocampus that differ from SST. To uncover the cellular mechanisms affected by CST, we studied the electrophysiological postsynaptic effects of CST and assessed its interaction with SST on hippocampal CA1 pyramidal neurons. CST altered intrinsic membrane properties and occluded SST effects, indicating that both peptides similarly augment the sustained K+ M- and leak-currents (IM and IK(L)). In the presence of SST, however, CST elicited an additional inwardly rectifying component in the hyperpolarized range. This effect was unaffected by barium, used to block K+ currents, but was completely prevented by the selective h-current (Ih) blocker ZD7288. CST, but not SST, selectively increased Ih in a concentration-dependent manner by augmenting its maximum conductance. CST did not shift the Ih activation curve, and the peptide effect was unaffected by a membrane-permeable analog of cAMP. We conclude that CST and SST similarly increase K+ conductances in hippocampal neurons, most likely by activating SST receptors. However, CST additionally augments Ih, a voltage-dependent current that plays a key role in the modulation of synaptic integration and regulates oscillatory activity. Our results indicate that CST targets a specific conductance unaffected by SST to modulate cellular mechanisms implicated in sleep regulation.

ATP modulation of excitatory synapses onto interneurons

Inhibitory interneurons play important roles in neuronal circuits, but the synaptic mechanisms that regulate excitatory input onto interneurons remain to be fully understood. We show that ATP-gated presynaptic P2X2 channels facilitate excitatory transmission onto stratum radiatum interneurons but not onto CA1 pyramidal neurons. ATP released endogenously during carbachol-induced oscillations facilitates excitatory synapses onto interneurons. Overall, these data provide evidence for the molecular identity, synaptic function, and interneuron synapse specificity of a presynaptic neurotransmitter-gated cation channel. The findings highlight a novel form of presynaptic facilitation for hippocampal interneurons and suggest a role for extracellular ATP in neuronal networks.

Novel insights into muscarinic acetylcholine receptor function using gene targeting technology

Muscarinic acetylcholine receptors (mAChRs) modulate the activity of an extraordinarily large number of physiological functions. Individual members of the mAChR family (M(1)-M(5)) are expressed in a complex, overlapping fashion in most tissues and cell types. However, the identification of the precise physiological roles of individual mAChR subtypes remains a challenging task because, with the exception of a few snake toxins, mAChR ligands that can activate or inhibit specific mAChR subtypes with a high degree of selectivity are not yet available. Knowledge of the specific roles of mAChR subtypes is of considerable interest for the development of novel, clinically useful mAChR ligands. In this article, recent studies of mutant mouse strains developed, using gene targeting techniques, to be deficient in one of the three G(q)-coupled mAChR subtypes (M(1), M(3) and M(5)) are discussed. These investigations have led to many important new insights into the physiological roles of these receptor subtypes.

Activation of muscarinic receptors in rat subfornical organ neurones

Cholinergic muscarinic inputs to subfornical organ (SFO) neurones in rats were studied using histochemical, molecular-biological and electrophysiological techniques. Neurones in the medial septum and the diagonal band (MS-DBB) were retrogradely labelled by a tracer wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex injected into the SFO. Some in the MS-DBB were double-labelled by choline acetyltransferase (ChAT) antibody. Many ChAT-immunoreactive fibres were observed in the SFO. M3 muscarinic receptor subtype-like immunoreactivity, detected using a polyclonal antiserum, was observed in the SFO. In slice preparations, muscarine induced inward currents in a dose-related manner. The inward currents were suppressed by the relatively M3 muscarinic receptor selective antagonist 4-diphenylacetoxy-N-methylpiredine methiodide. In the whole-cell current mode, muscarine depolarized the membrane with increased frequency of action potentials. Reverse transcriptase-polymerase chain reaction showed the presence of M2-M5 receptor mRNA in the SFO tissues. These results suggest that the SFO receives cholinergic muscarinic synaptic inputs from the MS-DBB. Acetylcholine postsynaptically activates and depolarizes neurones in the SFO partly through specific muscarinic receptors, including M3 receptor subtypes.

Gamma rhythmic bursts: coherence control in networks of cortical pyramidal neurons

Much evidence indicates that synchronized gamma-frequency (20-70 Hz) oscillation plays a significant functional role in the neocortex and hippocampus. Chattering neuron is a possible neocortical pacemaker for the gamma oscillation. Based on our recent model of chattering neurons, here we study how gamma-frequency bursting is synchronized in a network of these neurons. Using a phase oscillator description, we first examine how two coupled chattering neurons are synchronized. The analysis reveals that an incremental change of the bursting mode, such as from singlet to doublet, always accompanies a rapid transition from antisynchronous to synchronous firing. The state transition occurs regardless of what changes the bursting mode. Within each bursting mode, the neuronal activity undergoes a gradual change from synchrony to antisynchrony. Since the sensitivity to Ca(2+) and the maximum conductance of Ca(2+)-dependent cationic current as well as the intensity of input current systematically control the bursting mode, these quantities may be crucial for the regulation of the coherence of local cortical activity. Numerical simulations demonstrate that the modulations of the calcium sensitivity and the amplitude of the cationic current can induce rapid transitions between synchrony and asynchrony in a large-scale network of chattering neurons. The rapid synchronization of chattering neurons is shown to synchronize the activities of regular spiking pyramidal neurons at the gamma frequencies, as may be necessary for selective attention or binding processing in object recognition.

Regional brain uptake of the muscarinic ligand, [18F]FP-TZTP, is greatly decreased in M2 receptor knockout mice but not in M1, M3 and M4 receptor knockout mice

A muscarinic receptor radioligand, 3-(3-(3-fluoropropyl)thio) -1,2,5,thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine (fP-TZTP) radiolabeled with the positron emitting radionuclide (18)F ([(18)F]FP-TZTP) displayed regional brain distribution consistent with M2 receptor densities in rat brain. The purpose of the present study is to further elucidate the subtype selectivity of [(18)F]FP-TZTP using genetically engineered mice which lacked functional M1, M2, M3, or M4 muscarinic receptors. Using ex vivo autoradiography, the regional brain localization of [(18)F]FP-TZTP in M2 knockout (M2 KO) was significantly decreased (51.3 to 61.4%; P<0.01) when compared to the wild-type (WT) mice in amygdala, brain stem, caudate putamen, cerebellum, cortex, hippocampus, hypothalamus, superior colliculus, and thalamus. In similar studies with M1KO, M3KO and M4KO compared to their WT mice, [(18)F]FP-TZTP uptakes in the same brain regions were not significantly decreased at P<0.01. However, in amygdala and hippocampus small decreases of 19.5% and 22.7%, respectively, were observed for M1KO vs WT mice at P<0.05. Given the fact that large decreases in [(18)F]FP-TZTP brain uptakes were seen only in M2 KO vs. WT mice, we conclude that [(18)F]FP-TZTP preferentially labels M2 receptors in vivo.

Role of specific muscarinic receptor subtypes in cholinergic parasympathomimetic responses, in vivo phosphoinositide hydrolysis, and pilocarpine-induced seizure activity

Muscarinic agonist-induced parasympathomimetic effects, in vivo phosphoinositide hydrolysis and seizures were evaluated in wild-type and muscarinic M1-M5 receptor knockout mice. The muscarinic agonist oxotremorine induced marked hypothermia in all the knockout mice, but the hypothermia was reduced in M2 and to a lesser extent in M3 knockout mice. Oxotremorine-induced tremor was abolished only in the M2 knockout mice. Muscarinic agonist-induced salivation was reduced to the greatest extent in M3 knockout mice, to a lesser degree in M1 and M4 knockout mice, and was not altered in M2 and M5 knockout mice. Pupil diameter under basal conditions was increased only in the M3 knockout mice. Pilocarpine-induced increases in in vivo phosphoinositide hydrolysis were completely absent in hippocampus and cortex of M1 knockout mice, but in vivo phosphoinositide hydrolysis was unaltered in the M2-M5 knockout mice. A high dose of pilocarpine (300 mg/kg) caused seizures and lethality in wild-type and M2-M5 knockout mice, but produced neither effect in the M1 knockout mice. These data demonstrate a major role for M2 and M3 muscarinic receptor subtypes in mediating parasympathomimetic effects. Muscarinic M1 receptors activate phosphoinositide hydrolysis in cortex and hippocampus of mice, consistent with the role of M1 receptors in cognition. Muscarinic M1 receptors appear to be the only muscarinic receptor subtype mediating seizures.

Mechanisms of gamma oscillations in the hippocampus of the behaving rat

Gamma frequency oscillations (30-100 Hz) have been suggested to underlie various cognitive and motor functions. Here, we examine the generation of gamma oscillation currents in the hippocampus, using two-dimensional, 96-site silicon probes. Two gamma generators were identified, one in the dentate gyrus and another in the CA3-CA1 regions. The coupling strength between the two oscillators varied during both theta and nontheta states. Both pyramidal cells and interneurons were phase-locked to gamma waves. Anatomical connectivity, rather than physical distance, determined the coupling strength of the oscillating neurons. CA3 pyramidal neurons discharged CA3 and CA1 interneurons at latencies indicative of monosynaptic connections. Intrahippocampal gamma oscillation emerges in the CA3 recurrent system, which entrains the CA1 region via its interneurons.

Activation of muscarinic acetylcholine receptors enhances the release of endogenous cannabinoids in the hippocampus

Endogenous cannabinoids (endocannabinoids) are endogenous compounds that resemble the active ingredient of marijuana and activate the cannabinoid receptor in the brain. They mediate retrograde signaling from principal cells to both inhibitory ["depolarization-induced suppression of inhibition" (DSI)] and excitatory ("depolarization-induced suppression of excitation") afferent fibers. Transient endocannabinoid release is triggered by voltage-dependent Ca(2+) influx and is upregulated by group I metabotropic glutamate receptor activation. Here we show that muscarinic acetylcholine receptor (mAChR) activation also enhances transient endocannabinoid release (DSI) and induces persistent release. Inhibitory synapses in the rat hippocampal CA1 region of acute slices were studied using whole-cell patch-clamp techniques. We found that low concentrations (0.2-0.5 microm) of carbachol (CCh) enhanced DSI without affecting basal evoked IPSCs (eIPSCs) by activating mAChRs on postsynaptic cells. Higher concentrations of CCh (> or =1 microm) enhanced DSI and also persistently depressed basal eIPSCs, mainly by releasing endocannabinoids. Persistent CCh-induced endocannabinoid release did not require an increase in [Ca2+]i but was dependent on G-proteins. Although they were independent at the receptor level, muscarinic and glutamatergic mechanisms of endocannabinoid release shared intracellular machinery. Replication of the effects of CCh by blocking acetylcholinesterase with eserine suggests that mAChR-mediated endocannabinoid release is physiologically relevant. This study reveals a new role of the muscarinic cholinergic system in mammalian brain.

Reconstituted slow muscarinic inhibition of neuronal (Ca(v)1.2c) L-type Ca2+ channels

Ca(2+) influx through L-type channels is critical for numerous physiological functions. Relatively little is known about modulation of neuronal L-type Ca(2+) channels. We studied modulation of neuronal Ca(V)1.2c channels heterologously expressed in HEK293 cells with each of the known muscarinic acetylcholine receptor subtypes. Galphaq/11-coupled M1, M3, and M5 receptors each produced robust inhibition of Ca(V)1.2c, whereas Galphai/o-coupled M2 and M4 receptors were ineffective. Channel inhibition through M1 receptors was studied in detail and was found to be kinetically slow, voltage-independent, and pertussis toxin-insensitive. Slow inhibition of Ca(V)1.2c was blocked by coexpressing RGS2 or RGS3T or by intracellular dialysis with antibodies directed against Galphaq/11. In contrast, inhibition was not reduced by coexpressing betaARK1ct or Galphat. These results indicate that slow inhibition required signaling by Galphaq/11, but not Gbetagamma, subunits. Slow inhibition did not require Ca(2+) transients or Ca(2+) influx through Ca(V)1.2c channels. Additionally, slow inhibition was insensitive to pharmacological inhibitors of phospholipases, protein kinases, and protein phosphatases. Intracellular BAPTA prevented slow inhibition via a mechanism other than Ca(2+) chelation. The cardiac splice-variant of Ca(V)1.2 (Ca(V)1.2a) and a splice-variant of the neuronal/neuroendocrine Ca(V)1.3 channel also appeared to undergo slow muscarinic inhibition. Thus, slow muscarinic inhibition may be a general characteristic of L-type channels having widespread physiological significance.

H-channels in epilepsy: new targets for seizure control?

Hyperpolarization-activated cation channels (h-channels) are key regulators of neuronal excitation and inhibition, and have a rich diversity of subunit composition, distribution, modulation and function. Recent results indicate that the behavior of h-channels can be altered significantly by seizures. The activity-dependent, short-term and long-term plasticity of h-channels can, in turn, modulate neuronal excitability. The reciprocal interactions between neuronal activity and h-channels indicate that these ion channels could be promising novel targets for anti-epileptic therapies.

Multiple muscarinic acetylcholine receptor subtypes modulate striatal dopamine release, as studied with M1-M5 muscarinic receptor knock-out mice

A proper balance between striatal muscarinic cholinergic and dopaminergic neurotransmission is required for coordinated locomotor control. Activation of striatal muscarinic acetylcholine receptors (mAChRs) is known to modulate striatal dopamine release. To identify the mAChR subtype(s) involved in this activity, we used genetically altered mice that lacked functional M1-M5 mAChRs [knock-out (KO) mice]. In superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to concentration-dependent increases in potassium-stimulated [3H]dopamine release (by up to 60%). The lack of M1 or M2 receptors had no significant effect on the magnitude of these responses. Strikingly, oxotremorine-mediated potentiation of stimulated striatal [3H]dopamine release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly reduced (but not abolished) in M5 receptor KO mice. Additional release studies performed in the presence of tetrodotoxin suggested that the dopamine release-stimulating M4 receptors are probably located on neuronal cell bodies, but that the release-facilitating M5 and the release-inhibiting M3 receptors are likely to be located on nerve terminals. Studies with the GABA(A) receptor blocker bicuculline methochloride suggested that M3 and M4 receptors mediate their dopamine release-modulatory effects via facilitation or inhibition, respectively, of striatal GABA release. These results provide unambiguous evidence that multiple mAChR subtypes are involved in the regulation of striatal dopamine release. These findings should contribute to a better understanding of the important functional roles that the muscarinic cholinergic system plays in striatal function.